1. Field of the Invention
The present invention relates to a multistage phase shift mask, and more particularly, to a simplified method for fabricating a multistage phase shift mask for preventing a bridge pattern from generation at edge of an alternating phase shift mask.
2. Dissussion of the Related Art
FIG. 1 is a plan view of a conventional alternating phase shift mask and FIG. 2A is a cross-sectional view of the phase shift mask along a line A-A' in FIG. 1. Referring to FIGS. 1 and 2, a general phase shift mask 10 is formed by depositing a chrome thin film on a quartz substrate having a high light transmittance, patterning the same, selectively forming opaque chrome patterns 12, and forming a phase shift layer 13 stretching over an adjacent pair of chrome patterns 12. An amplitude of the light passing through the phase shift mask formed as described above is shown in FIG. 2B. The amplitude and intensity of the light incident onto a wafer through the phase shift mask are shown in FIGS. 2C and 2D, respectively. When using the phase shift mask 10 shown in FIG. 1, the phase of the light transmitting through the phase shift layer 13 is shifted by 180.degree. with respect to the light transmitted through the transmissive quartz substrate 11. The substantial intensity of the light incident onto the wafer is shown in FIG. 2C.
Therefore, if the photosensitive film of the wafer is exposed and developed using the phase shift mask 10, a good quality photosensitive pattern can be obtained in the direction of A-A' in FIG. 1.
FIG. 3A is a cross-sectional view of the phase shift mask along a line B-B' in FIG. 1.
The phase shift mask 10 has a structure such that the phase shift layer 13 is directly on the quartz substrate 11 in the direction of B-B' in FIG. 1. In the direction of B-B', the amplitude and intensity of a light incident onto a wafer through the phase shift mask are shown in FIGS. 3B and 3C, respectively. Therefore, if the photosensitive film of the wafer is exposed using the phase shift mask 10, a phase difference of 180.degree. from the light transmitted through the transmissive quartz substrate 11 and the light transmitted through the phase shift layer 13 as shown in FIG. 3B at an edge "E" of the phase shift layer 13 (FIG. 1) which contacts with the quartz substrate 11.
Therefore, a portion where the intensity of the light incident onto the wafer becomes zero is present, as shown in FIG. 3C. If a positive photosensitive layer is used, an undesired photosensitive layer is generated at a portion "E" of the phase shift layer shown in FIG. 1, which is called a bridge pattern.
The phase shift mask 10 has a problem in that an undesired bridge pattern is generated at the edge ("E" in FIG. 1) of the phase shift layer which directly contacts with the substrate between chrome patterns.
In order to solve such a bridge pattern problem of the alternating phase shift mask, a conventional stepped multistage phase shift mask, shown in FIG. 4, has been proposed.
FIG. 4 is a perspective view of a conventional stepped multistage phase shift mask.
Referring to FIG. 4, the conventional stepped multistage phase shift mask 20 is formed by stepwise etching a substrate a portion of which directly contacts with the edge of the phase shift layer 23, in order to solve the problem of a bridge pattern that is generated at the phase shift mask shown in FIG. 1. Through the conventional multistage phase shift mask, the phases are shifted to 60.degree., 120.degree. and 180.degree. at the portions corresponding to the light transmissive regions where the substrate directly contacts with the phase shift layer. Thus, a bridge pattern is prevented from being generated at "E." That is to say, by preventing the light intensity from becoming zero at "E," when the positive photosensitive layer is exposed and developed using the conventional stepped multistage phase shift mask, an unnecessary bridge pattern generation is prevented.
FIGS. 5A to 5H are cross-sectional views along a line C-C' in FIG. 4 showing a fabricating process of the conventional stepped multistage phase shift mask. FIGS. 6A to 6H are cross-sectional views along a line D-D' in FIG. 4 showing a fabricating process of the conventional stepped multistage phase shift mask.
The method for fabricating the conventional stepped multistage phase shift mask will now be described with reference to FIGS. 5A to 6H.
As shown in FIGS. 5A and 6A, a chrome thin film 22 is formed on a clean quartz substrate 21, a photosensitive layer 24A is deposited and patterned on the chrome thin film 22 to define a transmissive region T and shielding region S. At this time, the chrome thin film 22, which is the transmissive region, is exposed.
As shown in FIGS. 5B and 6B, the chrome thin film 22 of the transmissive region T is etched, using the photosensitive layer 24A as a mask. The chrome thin film 22 remaining in the shielding region S serves as a shielding layer. The remaining photosensitive layer 24A is removed.
As shown in FIGS. 5C and 6C, a phase shift layer 23 is deposited on the quartz substrate 21 containing the shielding layer and is etched so as to leave the phase shift layer 23 between an adjacent pair of shielding layers. At this time, in the direction of A-A', the edge of the phase shift layer 23 is formed on the shielding layer so that the edge of the phase shift layer 23 does not directly contact with the transmissive quartz substrate 21. However, in the direction of B-B', the edge ("E") of the phase shift layer 23 is not formed on the shielding layer so that the edge of the phase shift layer 23 directly contacts with the transmissive quartz substrate 21.
As shown in FIGS. 5D and 6D, a photosensitive layer 24B is deposited again on the whole surface of the substrate and is patterned so as to expose the portion "E" of the substrate 21 which is a transmissive region.
The exposed substrate is primarily etched be a constant depth using the photosensitive layer 24B as a mask. At this time, the light transmitting through the primarily etched portion 21A has a phase shift of 60.degree. with respect to the light transmitting through the unetched substrate.
As shown in FIGS. 5E and 6E, the photosensitive layer 24B is removed, and a photosensitive layer 24C is deposited again on the whole surface of the substrate. The photosensitive layer 24C is patterned so as to expose the primarily etched portion 21A. The exposed substrate is again etched to a constant depth using the photosensitive layer 24C as a mask. After the second etching, the photosensitive layer 24C is removed, as shown as FIGS. 5F and 6F. At this time, the light transmitting through the second etched portion 21B has a phase shift of 60.degree. with respect to the light transmitting through the primarily etched portion 21A, and thus has a phase shift of 120.degree. with respect to the light transmitting through the unetched substrate.
As shown in FIGS. 5G and 6G, a photosensitive layer 24D is deposited again on the whole surface of the substrate and is patterned so as to expose the second etched portion 21B. The exposed substrate 21B is again etched using the photosensitive layer 24C as a mask to obtain a multistage phase shift mask 20 of which the substrate being between an adjacent pair of chrome patterns are etched stepwise. At this time, the light transmitting through the third etched portion 21 has a phase shift of 60.degree. with respect to the light transmitting through the second etched portion 21B, and thus has a phase shift of 180.degree. with respect to the light transmitting through the unetched substrate.
FIG. 7A is a cross-sectional view of the multistage phase shift mask fabricated by the aforementioned process, and FIG. 7B shows the intensity of the light incident onto a wafer via the multistage phase shift mask shown in FIG. 7A.
According to the conventional stepped multistage phase shift mask 20, three steps of etching processes are executed for shifting phases, each with a phase difference of 60.degree., i.e., 60.degree. to 120.degree., and to 180.degree.. However, many more processes need to be executed for shifting phases with a less than 60.degree. phase difference.
In the case of the conventional stepped multistage phase shift mask 20, the portion where the phase is shifted with respect to the incident light is stepped. Hence, the phase is stepwise shifted to 60.degree., 120.degree., and 180.degree., thereby preventing a rapid decline in light intensity. Therefore, a bridge pattern is not generated, unlike the case in which a positive photosensitive layer is patterned using the phase shift mask shown in FIG. 1. However, in order to prevent the generation of the bridge pattern, a multistage substrate etching process needs to be executed. Thus, the process is complicated, and a misalignment of a photomask may occur.